Strand Insulation Research Articles

Modelling and experimental demonstration of a litz coil‐based high‐temperature induction heating system for melting application

This study presents analytical and numerical modelling and experimental demonstration of a litz wire-based induction heating system suitable for high-temperature application such as melting. The use of litz wire in induction heating coil minimises the skin effect and proximity effect and in turn improves the system efficiency. This makes it superior compared with a solid wire and copper tube induction coil. However, the limitation of litz wire is temperature withstanding capacity of the strand insulation material as a result of which it is rarely used in high-temperature applications (>700°C). Numerical modelling of induction heating process was done by using magnetic vector potential formulation and Fourier equations. Temperature-dependent material properties were taken account to precisely model the induction heating process. Analytically estimated and measured equivalent impedances of litz coil were compared at different frequency (1-20 kHz) for initial validation of simulation. The number of turns and frequency were selected as per required litz coil efficiency and induced power in crucible. Finally during the experimental demonstration, crucible and litz coil temperatures were measured by `K' type thermocouples and were compared with simulation results. The maximum temperature of ~750°C could be attained in the design while limiting the litz coil temperature within 85°C.

Improvement of stator insulation system for 24, 36 and 72 slots HV generators based on SNOPT method

Geometric properties of insulation layers in stator slot of HV generator such as semiconductive coatings, groundwall insulation, phase-phase insulation as well as turn and the strand insulation are major design consideration for insulation system of HV generators. In addition, electrical conductivity, thermal conductivity, heat capacity and electrical permittivity of these insulation layers have an important role on decreasing electrical field tensions. Obtaining of optimum value for these parameters leads to achieve less electrical field tensions and more uniform electrical field distribution across them. A very powerful method presented to optimization is SNOPT (generalpurpose system for large-scale nonlinearly constrained optimization) which is coupled with Finite Element Method (FEM). These processes of optimization have been done according the proposed optimization algorithm. In this algorithm the technical constraints have been considered. This paper describes the process used to perform improvement analysis of stator slot's insulation for 24, 36 and 72 slots generators with respect to objective function and constraints.

Plasticizer migration from PVC cable insulation – The challenges of extrapolation methods

A single strand PVC-P insulation including an internal metal conductor removed from the jacketed assemblies of a signal cable showed brittleness after 30 years service at 25 ± 3 °C in air. The PVC compound contained diisodecyl phthalate (DIDP), di(2-ethylhexyl) phthalate (DEHP) and a sizeable fraction of filler. Single strand insulation samples with internal metal conductor were aged in air at elevated temperatures for different periods of time after which the strain at break, the Young's modulus and the plasticizer content were assessed by tensile testing and liquid chromatography. Isothermal evaporation rates from pristine DIDP and DEHP and solutions of the two plasticizers were obtained by thermogravimetry. Data for Young's modulus, strain at break and plasticizer contents were extrapolated to service temperature using two different extrapolation methods, Arrhenius extrapolation (constant activation energy) and a method based on models by Langmuir, Clausius–Clapeyron and Kirchhoff. These methods assume that evaporation of plasticizers to the surrounding gas phase is the dominant deterioration mechanism. Both methods predicted only a minor decrease in plasticizer content after 30 years of ageing at 28 °C and thus a material with adequate mechanical properties. Liquid chromatography showed that the single strand cable samples contained a very low DIDP content (4 wt.%) and an anomalously high DEHP content; a finding that cannot be explained by the expected evaporative loss mechanism. It is suggested that DIDP was efficiently extracted by contact with a DEHP-rich interface at the insulation surface, a process which is active during plant operation, but could not be simulated by controlled laboratory accelerated ageing studies.

Magnetic and Transport AC Losses in HTS Roebel Cable

We present results for magnetic losses in a five strand HTS Roebel cable with 2 mm wide insulated strands. The loss as a function of field is compared to the isolated strands and an un-insulated cable. The loss as a function of the angle of the applied field to the cable normal is found to scale simply with the normal component of field. The dependence of the loss on the frequency of the applied field is presented and a small intrinsic frequency dependence of the superconductor is observed. The frequency dependence of the loss in a cable coupled by copper bridges is presented. The coupling loss is found to follow the Debye form at low fields. Transport AC losses in HTS Roebel cable with different strand insulation thicknesses are found to vary with the vertical separation of strands.

R&D of superconductors for 70 MW class generators

Super-GM has been developing 70 MW class superconducting generators (model machines) for a 200 MW class pilot generator. Three types of NbTi conductors were developed for each of three model machines. Regarding the high stability type conductors, residual resistance value of 6.5 /spl mu//spl Omega//m was achieved by adopting an Al stabilizer. Regarding the high current density type, Ic of 13.8 kA at 5 T was realized by reducing void fraction in conductors and maintaining high residual resistivity ratio. For the low AC loss type, high Jc and low AC loss (8.1 kW/m/sup 3/) have been succeeded by applying strand insulation and two stage cabling. To confirm superconducting characteristics concerning long-term reliability, Ic and AC loss of each conductor were measured under 10000 times repeated compressive force of 30 MPa that is supposed in daily start and stop operation of a generator for 30 years. In parallel to the R and D of NbTi conductors, R and D of Nb/sub 3/Sn conductors were also conducted for enhancement of critical current density and critical magnetic field.

Updated design of a 0.6 m bore 'wind and react' 12 T coil

The design of a 0.6-m bore wind-and-react Nb/sub 3/Sn solenoid was presented previously as part of the European R&D program on fusion magnets. Since then tests on the prototype base strands have been performed, with results showing that the specifications concerning J/sub c/ and AC losses were met, but the residual resistivity ratio (RRR) (273 K/20 K) was a factor of two lower than expected. By reducing the conduit thickness, the superconducting cable (cable-in-conduit conductor type) has been modified to accommodate an increased number of base strands, from 117 to 128 plus 16 high purity (OFHC) copper wires. A stress analysis of the new design has been performed, introducing into the ABAQUS code the elastic constants of each component of the CIC conductor. A sensitivity analysis of the quench behavior relative to different thermal disturbance durations and quenched conductor lengths has been performed using a 1-D code. Results of calculations as well as some experimental results concerning strand properties, cable manufacturing, and insulation tests are reported. >

Thermal Cyclic Degradation of Coil Insulation for Rotating Machines

Thermal cyclic degradation of epoxy micaceous insulation for ac high voltage generators and motors was investigated. From the experimental results, it is made clear as follows. Thermal cyclic degradation begins with a separation of the innermost main insulation layer from the strand insulation at the end of the iron core. It propagates in a mode of mica delamination and the formation of cracks. This degradation condition agrees well with the measurement results of non-destructive insulation tests using divided electrodes. The compatibility between a strand insulation and a main insulation greatly affects thermal cyclic degradation. Thermal degradation cannot be ignored under certain thermal cycle condition. Thermal cyclic degradation increases with an increase in the peak conductor temperature. Even in the case of a post impregnated insulation system where the coil insulation adheres to the iron core, the thermal cyclic degradation is little for a core length below 1.4 m and a 160 °C hottest conductor temperature. Non-destructive insulation diagnoisis of hydrogen filled water-cooled turbine-driven generator insulation shows little thermal cyclic degradation after approximately 15 years normal operations. Lastly, it is proposed that a thermal cylce test under voltage application is important hereafter, to grasp the degradation of actual machines precisely.

The use of stranded conductors to reduce Eddy losses in guideway conductors of high speed vehicles

It has been shown recently that the guideway conductors of high-speed trains will have to be segmented in order to keep the eddy losses induced by the large fields of the cryogenic magnets within reasonable limits. This paper describes an experimental program which has demonstrated that the normal stranding used in manufacturing aluminum and copper cables produces sufficient strand isolation, that strand insulation will not be required to keep the eddy losses small. The average measured difference in losses between cables having uninsulated strands and those having insulated strands was only 10 per cent for both the TE and the TM mode for both aluminum and copper cables.